Electrical switch contacts



Sept. 6, 1955 R, T FOLEY ET AL 2,717,296

ELECTRICAL SWITCH CONTACTS Filed Sept. 14, 1953 151155 Mfr/1L /l IML/@7707.5

esaegse Rober? 00P/1m @UPPER ox/f 0,717 R MC @arly United States PatentO ELECTRICAL SWETCH CONTACTS Robert T. Foley, Lanesboro, and Urin P.McCarty, Pittsfield, Mass., assignors to General Electric Company, acorporation of New York Application September 14, 1953, Serial No.379,844

4 Claims. (Cl. 200-166) This invention relates to electric switchcontacts and more particularly to electric switch contacts which operatewith a strong applied pressure forcing them tgether. Such contacts areused in tap changing switches .for changing the taps of a transformer.When used in an oil-filled transformer the switch is usually mountedbelow the surface of the insulating and cooling liquid with which thetank is substantially lled. A typical tap changing switch of thenonarcing duty tap changer type is described in Patent 2,255,501-Blume.While the present switch may be incorporated in a transformer of a typedescribed in the Blume patent it may also be used in certain inertatmospheres as will be more fully described hereinafter.

in a tap changer such as that described in the Blume patent pressurebetween surfaces in electrical contact is maintained by means of abiasing spring. Since these surfaces are in sliding contact muchdifficulty has been experienced in the past from rapid contact wear asthe result of galling of the surfaces coming into sliding contact witheach other. ln our copending application Serial No. 342,422, led March16, 1953, now Patent No. 2,680,174, dated June l, 1954, and assigned tothe same assignee as the present application, we described an electricalcontact surface which would not readily gall from sliding Contact. Thisconsisted of a thin film of tin oxide overlying a coating of tinelectrodeposited upon the base material of the contact. While the tinoxide coated contacts described and claimed in our copending applicationare entirely satisfactory we have now discovered a more easilyfabricated contact which will also resist galling during pressuresliding contact. This present switch can be used in the same electricalsystem as the above described Blume type switch.

lt is an object of the present invention to provide non-galling contactsurfaces which are easily produced.

lt is another object of the invention to improve the operation of tapchanging switches in which the contact members are normally requiredslidably to engage one another at pressures of from to 75 pounds.

The present invention is based on the discovery that ordinary coppercontacts will not gall from sliding contact provided a very thin coatingof copper oxide is applied to the contacting surfaces. Briefly stated,in accordance with one of its aspects, the invention is directed to anelectrical contact comprising a conducting base, and a layer of copperoxide 0.0020.008 mil in thickness overlying the base, the layer ofcopper oxide serving as a contacting surface.

Fig. l of the drawing is a broken perspective View partly' in section ofa load ratio adjuster switch; Fig. 2 is a side elevation view partly insection of the switch shown in Fig. l; and Fig. 3 is an enlargedsectional view of a single set of switch contacts.

Referring now to the drawing and more particularly to Fig. l, there isshown a portion of a tap changer or load ratio adjuster switch utilizedto change the taps 2,717,296 Patented Sept. 6, 1955 of a transformer(not shown). These switches, which normally operate immersed in oil,have a center support post 10 terminating in a stationary circularcontact plate 11. Coaxial with the support post 10 is a rotatable crankor selector shaft 12 on which is mounted a crank arm 13 terminating atits outer end in a pair over-and-under radial slots 14 and 14a,respectively, which maintain a pair of spaced Contact supports 15 in aradial position. The contact supports 15 serve as bases for mounting apair of clamping contacts 16 at their inner end and clamping contacts 17at their outer end. The inner clamping contacts 16 are in contact withthe contact plate 11 and the outer clamping contacts 17 make selectiveContact with one of a plurality of stationary contact terminals 18.

Clamping pressure by the contacts 16 against the contact plate 11 andcontacts 17 against a particular one of the terminals 18 is maintainedby a spiral compression biasing spring 19 mounted upon a bolt Z1extending in free movement through the contact supports 15. The spring19, which desirably exerts a pressure of ten pounds or more against theContact supports 15, works against the head of the bolt 21 and a washer22 maintained on the opposite end of the bolt 21 by a Cotter pin 23.

The operation of the switch is as follows: assume that shaft 12 isrotated clockwise as viewed from above. This will cause the crank arm 13to rotate clockwise as viewed in Fig. l thus causing the assembly whichis carried by the crank arm to advance the contacts 17 to the nextterminal 18. Since a current in excess of 400 amperes is normallycarried through the contacts it has been past practice to provide thecontacting surfaces with a very smooth linish to insure low-resistancecontact and reduce sliding friction to a minimum. As previouslymentioned, such contacts are subject to severe galling. We havediscovered that this galling can be greatly minimized by providing thecopper Contact surface with a film of copper oxide of controlledthickness. A lm applied in accordance with our invention will notincrease the Contact resistance to a noticeable degree.

In Figs. l and 2 the heavy shading of the contact surface indicates acoating of copper oxide. All contacting surfaces are provided with suchcoatings. These coatings are shown more clearly in Fig. 3 though theirthickness has been greatly exaggerated for purposes 0f clarity ofillustration. Also shown in Fig. 3 is an underlying layer of copper overa base metal. lf the base metal is composed of copper, as is normallythe case, there will be no separate copper layer as shown in Fig. 3.However, the base metal may be a less expensive metal than copper inwhich case it is necessary to coat it with copper before applying thecontacting coating of copper oxide.

lt has always been thought that the best copper contacting surface wasone highly polished and subjected to a wiping action during operationwhich would prevent the formation of a surface film. it was thought thatnothing could be done to prevent the galling of such highly polishedsurfaces during sliding contact under pressure. We have discovered thatgreat resistance to galling is conferred upon copper contacts which aregiven a copper oxide coating of a thickness of 0.002 mil to 0.008 mil.This thin coating may be described as that which appears on a polishedsurface as a slight dull bloom. The method of applying the coating isnot important as long as the coating furnished is continuous and withinthe range of thickness set forth above. A chemical bath may be used toprovide the oxide coating or the piece to be coated may be exposed for ashort period of time to the action of an oxidizing furnace at raisedtemperature. Both of these means of applying an oxide coating are sowell known to those skilled in the art that they will not be describedfurther herein. An oxide coating of the specied thickness has anegligible effect on the Contact resistance; yet such coated contactshave resisted galling at a contact pressure as high as 30 pounds whensevere galling occurred with non-oxidized surfaces subjected to acontact pressure of l5 pounds. In view of the small Contact area thisrepresents a pressure in excess of several hundred pounds per squareinch.

According to electron diffraction examination our copper oxide films arecomposed of both cuprous oxide and cupric oxide. While other substancesare no doubt present in trace quantities it is desirable to have theoxide film composed substantially of the oxides of copper solely.

A short exposure to air will not seriously affect the contact surfacesand where the contacts are arranged to have sul'icient abrasive actionon each other to remove any film which may form thereon prolonged use inair is possible.

The contacts of this invention may be used in inert media such as oil oratmospheres consisting of nitrogen or the rare gases. Such media providesurrounding environments which are neither oxidizing nor reducing tocopper. Performance is also satisfactory where the contacts aresubmerged in a dielectric oil and exposed to an inert atmosphereperiodically.

While there has been shown and described a particular embodiment of theinvention, it will be obvious to those skilled in the art that changesand modifications, can be made without departing from the invention;therefore, it is aimed in the appended claims to cover all such changesand modifications as fall within the true spirit and scope of theinvention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. An electrical contact member comprising a conducting base, and alayer of copper oxide 0002-0008 mil in thickness overlying said base,said layer of copper oxide serving as a contacting surface.

2. In a switch having co-operating contacts adapted to slidably engageone another at pressures of 10 t0 pounds, switch contacts comprising aconducting base, and a layer of copper oxide 0002-0008 mil in thicknessoverlying said base, saidlayers of copper oxide serving as thecontacting surfaces of said switch contacts.

3. An oil immersed switch comprising co-operating switch contactsadapted to slidably engage one an0ther at pressures of 10 to 75 pounds,the surfaces of said contacts which are engaged by one another havingthereon a coating of copper oxide having a thickness of from 0002-0008mil.

4. A nonarcing duty tap changer switch for use in an inert mediumcomprising, in combination, a plurality of stationary Contact memberssymmetrically distributed around a common axis, and a movable con tactmember for selective slidable contact with stationary contact members atcontact pressures of from l0 to 75 pounds, the contact surfaces of saidstationary contact members and said movable Contact member havingthereon thin surface coatings of copper oxide o a thickness of from0002-0008 mil.

References Cited in the file of this patent UNITED STATES PATENTS

